Fix timezone parsing functions for non-hour timezones

The `utctz_to_altz` and `altz_to_utctz_str` functions fail to handle
timezones with UTC offsets that are not a multiple of one
hour. Rewrite them and add some unit tests.

Fixes #630

2 files changed, 40 insertions, 16 deletions

diff --git a/git/objects/util.py b/git/objects/util.py
index f405d628..af279154 100644
--- a/git/objects/util.py
+++ b/git/objects/util.py
@@ -137,22 +137,25 @@ def get_object_type_by_name(
 
 
 def utctz_to_altz(utctz: str) -> int:
-    """we convert utctz to the timezone in seconds, it is the format time.altzone
-    returns. Git stores it as UTC timezone which has the opposite sign as well,
-    which explains the -1 * ( that was made explicit here )
-
-    :param utctz: git utc timezone string, i.e. +0200"""
-    return -1 * int(float(utctz) / 100 * 3600)
-
-
-def altz_to_utctz_str(altz: float) -> str:
-    """As above, but inverses the operation, returning a string that can be used
-    in commit objects"""
-    utci = -1 * int((float(altz) / 3600) * 100)
-    utcs = str(abs(utci))
-    utcs = "0" * (4 - len(utcs)) + utcs
-    prefix = (utci < 0 and "-") or "+"
-    return prefix + utcs
+    """Convert a git timezone offset into a timezone offset west of
+    UTC in seconds (compatible with time.altzone).
+
+    :param utctz: git utc timezone string, i.e. +0200
+    """
+    int_utctz = int(utctz)
+    seconds = ((abs(int_utctz) // 100) * 3600 + (abs(int_utctz) % 100) * 60)
+    return seconds if int_utctz < 0 else -seconds
+
+
+def altz_to_utctz_str(altz: int) -> str:
+    """Convert a timezone offset west of UTC in seconds into a git timezone offset string
+
+    :param altz: timezone offset in seconds west of UTC
+    """
+    hours = abs(altz) // 3600
+    minutes = (abs(altz) % 3600) // 60
+    sign = "-" if altz >= 60 else "+"
+    return "{}{:02}{:02}".format(sign, hours, minutes)
 
 
 def verify_utctz(offset: str) -> str:
diff --git a/test/test_util.py b/test/test_util.py
index 90dd89a9..c17efce3 100644
--- a/test/test_util.py
+++ b/test/test_util.py
@@ -333,6 +333,27 @@ class TestUtils(TestBase):
         self.assertRaises(IndexError, ilist.__delitem__, 0)
         self.assertRaises(IndexError, ilist.__delitem__, "something")
 
+    def test_utctz_to_altz(self):
+        self.assertEqual(utctz_to_altz("+0000"), 0)
+        self.assertEqual(utctz_to_altz("+1400"), -(14 * 3600))
+        self.assertEqual(utctz_to_altz("-1200"), 12 * 3600)
+        self.assertEqual(utctz_to_altz("+0001"), -60)
+        self.assertEqual(utctz_to_altz("+0530"), -(5 * 3600 + 1800))
+        self.assertEqual(utctz_to_altz("-0930"), 9 * 3600 + 1800)
+
+    def test_altz_to_utctz_str(self):
+        self.assertEqual(altz_to_utctz_str(0), "+0000")
+        self.assertEqual(altz_to_utctz_str(-(14 * 3600)), "+1400")
+        self.assertEqual(altz_to_utctz_str(12 * 3600), "-1200")
+        self.assertEqual(altz_to_utctz_str(-60), "+0001")
+        self.assertEqual(altz_to_utctz_str(-(5 * 3600 + 1800)), "+0530")
+        self.assertEqual(altz_to_utctz_str(9 * 3600 + 1800), "-0930")
+
+        self.assertEqual(altz_to_utctz_str(1), "+0000")
+        self.assertEqual(altz_to_utctz_str(59), "+0000")
+        self.assertEqual(altz_to_utctz_str(-1), "+0000")
+        self.assertEqual(altz_to_utctz_str(-59), "+0000")
+
     def test_from_timestamp(self):
         # Correct offset: UTC+2, should return datetime + tzoffset(+2)
         altz = utctz_to_altz("+0200")